Molecular motion in the crystalline donor–acceptor complex trimethylamine–trifluoroborane as studied by proton and fluorine magnetic resonance

Abstract

This paper is dedicated to Professor Charles A. McDowell on the occasion of his 70th birthday YUKIKO N. CHIHARA, NOBUO NAKAMURA, and HIDEAK~ CHIHARA. Can. J. Chem. 66, 1848 (1988). The proton and fluorine second moments and the spin-lattice relaxation times TI (at 25.75 MHz) were measured in the solid complex trimethylamine-trifluoroborane from 53 to 300 K. There is a cross-relaxation effect between 'H and 1 9~ below 120 K. Their TI minima near 140 K led to activation energies of 10.6 and 11.1 kJ mol-I for the reorientation of the methyl and the trifluoride groups, respectively. An additional minimum observed in the TI of protons at about 240 K was attributed to the reorientation of the trimethylamine group about the N-B bond with an activation energy of 25.0 kJ mol-I. A comparison of these potential parameters with those in other trimethylamine complexes led to a notion that the activation energies for the reorientation of the (CH3)3N group in the trimethylamine-trifluoroborane complex are mainly governed by the intramolecular steric effect. Trimethylamine (TMA) forms donor-acceptor complex compounds with a variety of electron accepting materials (1). Some of these complexes have threefold symmetry and different types of intramolecular degrees of freedom. Moreover, some undergo a phase transition or transitions in their crystalline state (2-4) but the nature of the phase transitions has not yet been clarified. To understand the mechanism of phase transition(s) as well as the intricate inter-or intramolecular interactions in the crystals of these molecular complexes, studies are necessary both of the dynamical properties of molecules and of the crystal lattices in relation to detailed structural studies.The 1:l complex compound between TMA and trifluoroborane (TFB) is a typical example of such a complex. It has a molecular threefold symmetry axis and three intramolecular rotational degrees of freedom, and can undergo two different rotations as a whole, i.e., the rotations about the molecular symmetry axis and about the two short axes. The molecule has a large dipole moment (5.76 D) (5) and the crystal belongs to a rhombohedra1 space group with three TMB .TFB complexes per unit cell (6). This compound does not undergo any phase transition between 77 K and its melting point and therefore is a good specimen for examining molecular motion in detail without interruption by the discontinuities of a phase transition. This paper presents the results of proton and fluorine nuclear magnetic resonance and relaxation measurements below room temperature. The molecular modes are assigned and the cross-relaxation effect between the proton and fluorine is discussed. The origin of the activation processes and the possible mechanism of phase transitions in trimethylamine complexes will be considered. ExperimentalTMA-TFB was synthesized according to a method reported in the literature (7); a saturated aqueous solution of KOH was added to trimethylamine hydrochloride from a commercial source and gaseous TMA thus generated was intro...